JuanAntonio, 

What kind of event are you trying to invoke?

From the client event documentation;

A client event is that point in the process that requires some human interaction

1. 
   
2. What needs to be done?
   
3. The sequence in which it needs to happen?
   
4. Who needs to be involved (participants) - and at what stage?
   
5. What data the event needs?
   
6. The sequencing and the participants are particularly important - as they provide clues as to when multiple events can be added to the same activity.
   

From the How to use Asynchronous Server Events in Blackpearl:

Asynchronous Server Events provide a mechanism for stopping process flow to wait for an external application to programmatically finish the server event before process flow resumes.

Are those two your only options, I've seen other ways to interact with external services;

https://www.k2.com/onlinehelp/k2blackpearl/userguide/current/webframe.html#walkthrough_create_external_content_type_(secure_store).html

Thank you